Implantable or insertable nuclear magnetic resonant imaging system

ABSTRACT

Nuclear Magnetic Resonant Imaging (also called Magnetic Resonant Imaging or “MRI”) devices which are implantable, internal or insertable are provided. The disclosure describes ways to miniaturize, simplify, calibrate, cool, and increase the utility of MRI systems for structural investigative purposes, and for biological investigation and potential treatment. It teaches use of target objects of fixed size, shape and position for calibration and comparison to obtain accurate images. It further teaches cooling of objects under test by electrically conductive leads or electrically isolated leads; varying the magnetic field of the probe to move chemicals or ferrous metallic objects within the subject. The invention also teaches comparison of objects using review of the frequency components of a received signal rather than by a pictorial representation.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of copendingapplication Ser. No. 12/570,611, filed on Sep. 30, 2009, the contents ofwhich are incorporated by reference in their entirety herein.

FIELD OF THE INVENTION

The present invention generally relates to Nuclear Magnetic ResonantImaging (also called Magnetic Resonant Imaging or MRI), and moreparticularly, to a method and apparatus for improving image generation,calibration, cost, efficiency, and associated techniques for NuclearMagnetic Resonant Imaging.

BACKGROUND

MRI was founded on the notion that precise differentiation of nucleonprecession frequencies was possible via magnetic gradients and RadioFrequency (RF) activity. The many device expositions of MRI, althoughformidable, are based on evolving engineering practices in signalprocessing and electromagnetic (EM) activity. Currently, a largemacro-engineering MRI may give the best pictorial representation formost applications for the foreseeable future.

Briefly stated, the “classic” operation of a traditional MRI includesthe following functionality:

A “significant” magnetic field aligns the electronic spins and someportion of the photonic hydrogen or other molecules in a patient;

Additional fields may have magnetic gradients in one or more axes;

Radio Frequency (RF) transmitter perturbs the spins of thispreviously-aligned field, in one or more times and uses a variety ofgradient practices;

RF receiver records lifecycles of the perturbed spins; and

Processor collates computes and displayed the perturbed spin lifecycle,and translates them into a spectroscopic or image representation.

Most MRI devices have therefore been “outside in”; they are generallylarge objects wherein the subject under test is placed inside them.These “outside in” systems have traditionally been extremely large,heavy and expensive. Thus, they cannot be readily used in field ormobile diagnosis, nor can they be implanted or inserted in human orveterinary bodies.

A lesser number of MRI devices are “inside out”; they contain themagnetic and electrical probes which are insertable, internal andimplantable in the subjects under test. Such subjects may range fromlarge geological structures, to manmade objects, plants, animals, andhumans. However, the current MRI devices that are “inside out” are proneto inaccuracy, overheating, and are expensive.

Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY

In one aspect of the invention, a method is implemented for analyzingthe structure and operations of an object under examination by magneticresonant imaging, comprising the steps of: injecting, inserting orplacing a plurality of reference object calibration targets of knownsize, geometric shapes, and magnetic profile, into or adjacent to anobject under examination, at a plurality of locations; obtaining animage or radio frequency spectrum analysis of said object underexamination by magnetic resonant imaging; and refining the image orradio frequency spectrum analysis obtained of such object underexamination by enlarging or shrinking it in one or more lineardimensions, to conform to the characteristics of said reference objectcalibration targets.

In another aspect of the present invention, a method is provided forcooling an insertable, movable, or implantable MRI system comprising thesteps of: inserting or moving an MRI device; cooling the insertedportion by use of an extrinsic cooling unit leads which remove heat fromthe inserted unit by thermal conduction, convection or radiation.

In yet another aspect of the present invention, a method is provided forconducting the comparative analysis of the structure and operations ofan object under examination by magnetic resonant imaging, comprisingcomparing frequency-scans of the received signal without furtherprocessing of the signal to prepare a constructive image

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows an internal, implantatable or insertable MRI device withsolid or stranded electrically-conducting cooling leads.

FIG. 2 shows an internal, implantatable or insertable MRI device withsolid or stranded electrically-conducting cooling leads.

FIG. 3 shows an internal, insertable or implantable MRI device withliquid, electrically conductive cooling leads.

FIG. 4 shows an internal, insertable or implantable MRI device withliquid conductive cooling.

FIG. 5 shows an inserted geometric calibration object which is used forcalibration of the radio-frequency received images.

FIG. 6 shows a method of calibrating the visible image to one or moreinserted calibration targets.

FIG. 7 shows an internal, insertable or implantable MRI device wherecomparisons of the object-under-test are made by radio-frequencyspectrums analysis.

FIG. 8 shows infusion, migration, diffusion or leaching ofmagnetic-relevant chemical elements, or solid ferrous objects.

DETAILED DESCRIPTION

The present invention generally relates to Nuclear Magnetic ResonantImaging (MRI), and more particularly, to a method and apparatus forimproving image generation, calibration, cost, efficiency, andassociated techniques for Nuclear Magnetic Resonant Imaging. Morespecifically, the invention improves an “inside-out” imaging device. Thepresent invention can be used in a range of different applications suchas, for example, diagnosis and monitoring of body functions, asdiscussed in more detail below.

In classical MRI, a large device has the luxury of an environment thathas stability in many areas, including physical, environmental,shielding, electronic and magnetic. On the other hand, a miniature MRImay have advantages stemming from its small size. Its modest dimensionslend itself to internal stability. Thus, one can leverage theminiaturization to an advantage; and at least have rigidity in theimplantation there. In addition, an implantable MRI has close—indeeddirect—tissue contact. Therefore, although the Larmor frequency per-serequires high magnetic flux density, the RF needed to perturb the fieldmay be smaller, and the received portion of the perturbed spins may behigher. Also, as the “leash” of MRI control cables can be veryaccurately designed and measured, it is possible to control phase verywell. Further, although large high-Tesla magnets, both permanent andelectro-resistive, require substantial infrastructure, miniaturizedmagnets in orthogonal dimensions can control a local magnetic field verywell.

In embodiments, the present invention may use dynamic RF tailoring andtuning to compensate for difficulties in field congruity. In furtherembodiments, the present invention may use the same solenoid windingsfor DC magnetic field instantiation, both prime and gradient, as well asfor AC RF generation and reception. Furthermore, the present inventionmay use analog scanning and spread-spectrum reception and transmission,to supplement or augment digital Fourier transform.

As will be appreciated by one skilled in the art, the present inventionmay take many forms of a “traveling” MRI in the object under test, suchas, for example, similar to electronic lozenges used in endoscopy orcolonoscopy. The present invention may also be assimilated with aninsertable, implantable or internal MRI. The present invention may alsouse one or more injected, inert quality-control micro-targets forcalibration. Additional embodiments are also provided for some potentialusability in specific situations.

The present invention may use extremely small injectable, implantable,or attached ferrous spheres or other shapes as “targets” for systemcalibration. These can provide external readout of the magnetic shape,or can deform externally or internally by one-time or repeatable fielduse. In further embodiments, for an insertable or implanted MRI asdescribed in the present invention, the MRI can be calibrated to aid inpost-processing by active use of small injected or inserted microtargets, not only for calibration but whose possible evolution undercertain chemical, thermal, or acoustic circumstances itself may be anindication of body function.

The present invention also can use low-cost signal processing byspectrum analyzer, as opposed to, or in conjunction with, transforminginto pictorial representation. Also, embodiments of the presentinvention vary the magnetic field to push, infiltrate, and circulatemagnetic therapeutic agents. Also, in embodiments, the present inventionadjusts the cooling of the MRI device to conform to the magneticactivity.

In embodiments, the MRI need not be “on” continually, nor need it betotally internally self-contained. For example, the MRI of the presentinvention may conceptually be “on demand” or of specializedutility—e.g., tuned for spectroscopic analysis and differentiation ofdisease or healing products, in highly localized body areas. There arevarying situations where a “constant” MRI may be useful, being on all ormost of the time. But just as with a cardiac pacemaker, the fullfunctionality of the MRI need not be always in use. It may be sufficientto have some vestigial or highly tuned functionality. In embodiments,the present invention may use external physical or inductive chargingand tuning by patient or remote-via-modem to healthcare provider.Indeed, similar to ambulatory telemedicine, the present invention maytake the form of a remote data collation suite where one experthealthcare provider monitors many implanted MRI. It is contemplated thatimplantation/insertion is for limited periods of time.

Not all components of the MRI need be co-located in the body.Accordingly, in embodiments, various components can be distributedinternal and slightly sub-dermal, for example.

In the individual clinical situations, possible uses of an insertable orimplanted MRI include brain and heart monitoring, and diabetes controland functionality. An MRI introduced under light sedation analogous to acolonoscopy or endoscope can be used for localized analysis of theefficacy of highly-potent infused drugs with magnetic markers. Inembodiments, the MRI use of the present invention may occur by equipmentlogic or by the patient him/herself turning on the device during periodsof perceived pain, etc.

MRI base functionality might further be used to slowly “leech” ferrousparticles or drugs from a patient, or quickly use magnetic, as opposedto electric elements for cardiac rhythm renormalization. The MRI couldbe used, but in extreme situations, with pulsed MRI high magnetic fieldsused to deflect received ferrous metal munitions away from core bodyareas, as well as egress any shell fragments in a controlled matter.Although internal heating may otherwise be done by diathermy or otherdevices, this invention permits the infusion, dissemination or movementof chemicals by the same device which performs the analysis.

Although magnetic fields big and small may be difficult to directlycontrol, RF fields are very amenable to control in frequency (1×10⁻⁹),phase, slew rate, and interference patterns. The MRI takes advantage ofthis by active electronics and continual tuning. In addition, in certaindefined two dimensional circumstances, digital artifacts can beeliminated by a completely analog control unit.

Rather than pictorial representation, the present invention uses Fouriertransform, or panoramic or spectrum analysis by an analog ramp generatorand controlled phosphor persistence. This can be used for spectroscopicor low-resolution imaging, as well as for very accurate tuning of thelocal environment prior to FFT/DFT being established.

Magnetic Field Challenges—Generation

The central aspect of traditional MRI is proton spin and resonance. TheLarmor resonant frequency can be described as. ω=γB. For protons, γequal 42.5775MHz/Tesla. Clearly, the resonant frequency is directlyproportional to the frequency used: 1 Tesla 42.6 MHz, 10 Tesla 426 MHz,1×10 -2 Tesla .42 MHz, etc. Thus, lower magnet strength would need alower frequency.

The present invention can use known isotope or nucleon for efficiency.For example, Hydrogen 2H which is already among the highest ingyromagnetic ratio, can be implemented with the invention. Also, 3Hwhich is slightly higher at 45.4158 MHz/Tesla can be used, but this hasalmost no abundance. Also, as to what is the strongest magnet whichcould be implanted or inserted, a threshold is the size ofalready-implanted types of devices, both one-time and free ranging.These known devices include, for example, cardiac pacemakers, insulinpumps, colonoscopy and endoscope devices, etc. Based on these devices,it is contemplated that 5 cm×2 cm device is appropriate. Also, thepresent invention proposes a “lozenge” shaped device with external powerand control cabling. Using either Neodymium Iron Boride or Alnico D5, itis possible to obtain a small, local, permanent magnet for the mainfield of over 1 Tesla.

The present invention contemplates both permanent magnet and solenoidresistive electromagnets. Accordingly, although both permanent andresistive electromagnets are possible, for initial human use, it iscontemplated that a permanent magnet for the main field be used andresistive electromagnets for the gradient field. It is contemplated,though, that a resistive electromagnetic for the initial field may beused after further testing.

As noted, a single loop of wire can produce a flux density ofapproximately:

$B = \frac{\mu\;{la}\; 2}{2\left( {{a\; 2} + {\left( {z - s} \right)2}} \right){3/2}}$

Which can be reduced to the following for a solenoid:

$B = \frac{\mu\;{{nl}\left( {{\cos\;{\alpha 1}} - {\cos\;{\alpha 2}}} \right)}}{2}$

In the above well-known equations, “B” is the magnetic field magnitudein Telis; “μ” is the relative permeability of the material in the core;n is the number of turns; “I” is the current through the wires inAmperes; “a” is the diameter of the solenoid; “z” is the long axis ofthe solenoid; “α1” is the angle of the wiring turn compared to the longaxis “z”; and “α1” and “α2” are the angles from the long axis “z” to thediameters at the solenoid ends, from any point on the “z” axis.

An issue that may arise in an insertable MRI is the resistive heatingI²R of the solenoid wire. Although the body's blood system continuallycarries away internal heat loads at a temperature of around 303° k, thisis typically for a healthy individual. Although the present inventioncontemplates the use of a pulsed MRI to reduce heating, in a non-pulsedcase it may be necessary to thermodynamically transfer significantenergy from wire resistance, as well as from integral magnetic and RFtissue heating. As estimation, it may not be possible to countenance anet differential additional thermal load of more than 50 Joules, in thiscase 10 watts for five seconds. This would be distributed over the areaof the lozenge, and if the heat dissipation was more-or-less uniform, noinjury might be expected. In any event, it is contemplated that the MRIwould include a thermostat to include rapid shutdown in an emergency.

Although tissue heating may be an issue, an electrical field magnet iscontemplated for future implementation, particularly with a largerlozenge. It is also possible to include as necessary inward orin-and-out water cooling to the MRI “lozenge” and surrounding tissue.This could, for example, encompass calibrated flow, which could involvenot only heat transfer and cooling, but metered velocity flow forimaging, or die or artifact introduction. The present invention alsocontemplates very tailored short field pulses, which although carryingthe same or higher power (watts) would have less total energy (Joules)and could be thermally dissipated and mathematically analyzed quickly.

Even for the field solenoids, there are of course problems inhysteresis, eddy currents and core saturation. However, these may beaddressed or compensated for as noted in the present disclosure. Oneembodiment of doing this is post-processing, by adjusting and stretchingthe raw image, to conform the image representation to calibrationtargets noted in the present disclosure. Also, the magnetic fields neednot be perfect, or even homogenous. For incongruities can be resolved inpost-processing, particularly if there is careful analog tuning andquality control calibration phantoms.

As an example, assume use of a 1.1 Tesla magnet, which provides anLarmor Frequency of around (1.1×42.6)=46.9 MHz for Hydrogen. (It isnoted that 46.9 MHz is well within the US FCC Radio allocation of “LandMobile” (30-50 MHz), for which outside transmitters are relatively fewand low powered). Thus, it is unlikely that the typical MRI will effector incur many problems. As the free speed of light is around 3×108m/sec,

$\lambda = \frac{C}{f}$

The resulting wavelength is about 6.2 meters, and a half wavelength fora dipole transmitter or receiver of about 3.1 meters.

However, the present invention does not need an antenna anywhere nearthis size. Instead, it is possible to use inductive coupling andconjugate matching to have a relatively efficient radiating andreceiving element which is much smaller. Although this may have a verylow radiation resistance, the present invention has a relatively smalltransmission line, and the overall efficiency is acceptable. Of course,this is all predicated on the fact that for MRI, a directional orsteerable antenna per se is not needed; what is primarily needed isaccurate frequency, phase and power control. Thus, MRI does not need avery large antenna capable of azimuthally correlation; rather, it is thepresence of the wave itself with gives the spatial conformance. Ofcourse, the smaller antenna may have lower input impedance, but the lowfrequency and short transmission line would ensure that little wasactually lost.

For the purposes of this invention, a representative configuration willbe a small insertable ovoid “lozenge” of approximately 5 cm×3 cm, whichis connected via a small diameter cable to a power supply/controllerunit (PSCU). The PSCU gives magnetic and RF power, and also is the RFreceiver. In the PSCU are the control and readout capability.

Per-se FFT signal processing capability is not always required. Much ofthis can be done pre-processing in the frequency domain, by directspectrum analysis or “panoramic” views by frequency sweeping. Forexample, on the output scope, the horizontal axis is the frequencyexpression; this is then swept by a saw tooth oscillator across theband. This may be swept say at 100 KHz, removing diminishing certainartifact situations.

By feeding AC and DC in various polarities, phases, series anddifferential ways, a thorough and subtle control of both RF and magneticfields can be created. The device per-se would preferably haveorthogonal elements, all fed through a narrow-diameter shielded cable tothe control unit.

A configuration of the present invention may use a permanent magnet. Inany event, gradients and RF transmitting and receiving control can beaffected to support either a permanent field magnet, or anelectromagnetically-derived field. Field uniformity is maintained andoptional power and cooling can be provided. Of course, when the MRIlozenge is out of a patient, very high powered pulses might be used toreform the magnetic structure of the permanent magnet, to providevirtual “shim” tuning should that be necessary according to the industrypractice known the ones skilled in the art.

FIG. 1 shows an internal, implantatable or insertable MRI device withsolid or stranded electrically-conducting cooling leads in accordancewith aspects of the invention. In embodiments, theelectrically-conductive leads are thermally conductive to relieve thethermal load of the subject under test. For example, an electrical wire110 of a material such as copper or other conductive material, whichconducts both electricity and heat, is electrically shielded by amaterial such as polystyrene 120. In embodiments, the wire 110 is placedin an implantable, internal or insertable MRI device 140. Inside the MRIdevice 140 are a plurality of solenoid windings 130, which may be in agenerally helix form. Although only one such winding, and one pair ofwires, is depicted schematically for clarity, those of skill in the artwill understand that the present invention contemplates more than onewinding and more than one pair of wires. The windings 130 perform one ormore functions of generating a magnetic field, varying the field,radio-frequency transmission and radio-frequency reception, for example.The heat generated with the insertable MRI device 140 may be deleteriousto the object under test. For this reason, a non-electricallyconductive, thermally-conductive cooled material such as ceramic 150 isin contact with or adjacent to one or more of the leads 110 to dissipatethe heat, e.g., act as a heat sink. The ceramic or other cooling element150 may itself be cooled by, convection, or radiation by an externalliquid or solid heat drain 160, by Pettier cooling or by othermechanisms to remove heat from element 150.

FIG. 2 shows an internal, implantable or insertable MRI device withsolid or stranded electrically-conducting cooling leads in accordancewith the invention. In embodiments of the invention, the electricalwiring 110 conducts electrical impulses to and from the inserted device,and the cooling is performed by a thermally conducting material 210adjacent to the insulation 120 of the wiring 110, which is extrinsicallycooled to remove thermal load at a position outside the device. Inembodiments, the cooling element 210 is thermally conducting, and may ormay not be electrically conducting. In other embodiments, the coolingelement 210 is adjacent to the outer wire insulation 120 and attached toan external liquid or solid heat drain 220.

FIG. 3 shows an internal, insertable or implantable MRI device withliquid, electrically conductive cooling leads in accordance with aspectsof the invention. In embodiments, the liquid carries the electriccurrents to the magnet and in addition is thermally conductive torelieve the thermal load of the subject under test. In embodiments,flexible, electrically insulated tubing 310 made of a material such asplastic is filled with a circulating, electrically-conductive liquid 320such as a saline solution in water. The electrically-conductive liquid320 is circulated through a heat-exchanger 350 within the MRI device350. In FIG. 3 as shown, there are two pairs of tubing, with the upperpair electrically isolated from the lower pair. An external electrode340 introduces current for magnetic or radio-frequency MRI use, and thisis carried by the respective electrically-conductive liquid 320 to aninternal MRI electrode 330 which powers the MRI solenoid 130.

FIG. 4 shows an internal, insertable or implantable MRI device withliquid conductive cooling, where the liquid does not provide electricalcurrent capability, but is thermally conductive to relieve the thermalload of the subject under test. In embodiments, a flexible, electricallyinsulated tubing 410 made of a material such as plastic is filled with acirculating liquid 420 such as a saline solution in water. The salinesolution 420 flows to a heat-exchanger cooling head 430 within the MRIdevice 140.

FIG. 5 shows an inserted geometric calibration object which is used forcalibration of the radio-frequency received images in accordance withthe invention. In this embodiment, one or more objects of knowngeometric size and shape 510 are inserted, injected, or implanted in theobject under test 520 to serve as references for pictorialrepresentation by the MRI device. 530. Although many shapes arecontemplated by the invention, spherical shapes objects provide the sameideal shape under any angle or orientation of viewing. The targets maybe inert, and/or bio-absorbable.

FIG. 6 shows a method of calibrating the visible image to one or moreinserted calibration targets. At step 610, the present invention insertsone or more geometric objects of known shape and size (e.g., dimension).For example, this can be one spherical target using a spherical target,on an X/Y axis viewer. At step 620, the present invention obtains an MRIimage. At step 630, the target image is evaluated as to correspondenceto known shape and size. At step 640, the present invention stretches orminimizes target image in various axes to conform to known shape andsize using a spherical target, on an X/Y axis viewer. In otherembodiments, optical or electronic image techniques can be used tostretch the image. This image may be stretched optically by lenses,electronically by a computer display using commonly-available imageprocessing software, or other means. After the image is stretched toconform to targets of known shape and size, it produces more accuraterepresentation of the image. At step 650, the present invention uses theresulting image.

FIG. 7 shows an internal, insertable or implantable MRI device wherecomparisons of the object-under-test are made by radio-frequencyspectrums analysis, not pictorial representation per-se. In thisexample, the traditional MRI proton ‘knock down and recover’ sequencesare performed in a conventional manner. At step 710, the presentinvention generates main magnetic field for rotational displacement. Atstep 720, the present invention generates additional gradient magneticfields. At step 730, the present invention applies RF energy to theobjects under examination. At step 740, the present invention receivesRF energy. In this implementation, the present invention bypasses apictorial representation and performs the processes of step 750. Inparticular, at step 750, the present invention displays RF waveformnon-pictorially on a two dimensional view, wherein one axis isfrequency, and another intensity. At step 760, the present inventionwill take later views of the same or other objects, and compare thefrequency vs. intensity display of the received RF energy. Theadvantages of not proceeding to a pictorial display are that the MRIequipment is much cheaper, and the receiver element can use simple sawtooth generators for frequency sweep. Although no pictorialrepresentation is used, the differential between two images can bediscerned by this depiction.

FIG. 8 shows infusion, migration, diffusion or leaching ofmagnetic-relevant chemical elements or solid ferrous objects by varyingthe magnetic field of the internal, insertable or implantable MRI devicein accordance with aspects of the invention. For example, magneticallysusceptible and/or magnetically charged solutions or particles 810 areinjected into the object under test 520. These particles 810 may bediffused throughout the object under test 520 by the magnetic elementsin the MRI device 530. As the object-under-test's thermal load may beparticularly high, embodiments of the present invention contemplateperforming this with a cooled MRI device, using one of the coolingtechniques previously described.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims, if applicable, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiment was chosen and described in order to best explain theprincipals of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated. Accordingly, while the invention has beendescribed in terms of embodiments, those of skill in the art willrecognize that the invention can be practiced with modifications and inthe spirit and scope of the appended claims.

What is claimed:
 1. A method for conducting a comparative analysis ofthe structure and operations of an object under examination by magneticresonant imaging, comprising comparing frequency scans of a receivedsignal without further processing of the signal to prepare aconstructive image.
 2. The method of claim 1, wherein the frequencyscans of the received signal is performed by a panoramic receiver. 3.The method of claim 1, wherein the frequency scans of the receivedsignal is performed by Fast Fourier Transform Analysis of a receivedwaveform.
 4. The method of claim 1, wherein the frequency scans of thereceived signal is performed by a tracking receiver tuned by a rampgenerator.
 5. The method of claim 1, wherein the method bypassespictorial representation, and uses Fourier transform, or panoramic orspectrum analysis by an analog ramp generator and controlled phosphorpersistence.
 6. The method of claim 4, wherein the ramp generator is ananalog ramp generator.
 7. The method of claim 6, further comprisingfrequency sweeping by a saw tooth oscillator across a frequency band. 8.The method of claim 7, wherein the frequency sweeping occurs at 100 KHzto remove artifact situations.
 9. The method of claim 8, wherein thefrequency scans are displayed in a two-dimensional view such that oneaxis of the two-dimensional view is frequency and another axis of thetwo-dimensional view is intensity.